Shallow source MOSFET
A semiconductor device comprises a drain, a body in contact with the drain, the body having a body top surface, a source embedded in the body, extending downward from the body top surface into the body, a trench extending through the source and the body to the drain, and a gate disposed in the trench, having a gate top surface that extends substantially above the body top surface. A method of fabricating a semiconductor device comprises forming a hard mask on a substrate having a top substrate surface, forming a trench in the substrate, through the hard mask, depositing gate material in the trench, where the amount of gate material deposited in the trench extends beyond the top substrate surface, and removing the hard mask to leave a gate structure that extends substantially above the top substrate surface.
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The present invention relates generally to semiconductor devices. More specifically, a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) device is disclosed.
BACKGROUND OF THE INVENTIONA trench power metal oxide semiconductor device is a common type of semiconductor device.
While this type of MOSFET device with recessed gate has proven useful, several problems remain. One of the problems associated with the current device structure is the on state resistance. Since the bottom portion of source region 104 is typically required to overlap with recessed gate 102 to insure proper device operation, the depth of source region 104 typically needs to meet a certain minimum. The current, which flows through the source region at a minimal required depth, leads to an on state resistance having a minimum value that is not easily reduced.
Another problem associated with the typical source depth requirement is gate capacitance. Since the channel typically requires a minimal channel length, a deeper source means that a deeper trench is typically required to accommodate the gate, thus increasing the gate capacitance and lowering the switching speed. The lateral diffusion of a deeper source typically requires a larger contact. As such, the reduction in device size and the increase in cell density are both limited.
Another problem associated with the current design is that the parasitic bipolar NPN transistor formed by source 104, body 106 and drain 108 is often easily turned on, thus limiting the operable range of the device.
It would be desirable to develop a MOSFET device with shallower source to improve cell density and to reduce on state resistance. It would also be useful if the resistance between the base and emitter of the parasitic bipolar transistor could be reduced so that the parasitic bipolar transistor would not be turned on easily. It would also be useful to develop a power MOSFET device with shallower trench depth and smaller gate capacitance to improve the device switching speed.
Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.
The invention can be implemented in numerous ways, including as a process, an apparatus, a system, a composition of matter, a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention.
A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.
A semiconductor device and its associated fabrication process are disclosed. The semiconductor device comprises a drain, a body in contact with the drain, a source embedded in the body, extending downward from the top surface of the body into the body, a trench extending through the source and the body to the drain, a gate disposed in the trench that has a gate top surface extending substantially above the source top surface, and a gate oxide insulating the gate from the source, the body and the drain. For the purpose of illustration, examples of MOSFET devices are discussed in detail throughout this specification. The techniques are also applicable to other device types such as Insulated Gate Bipolar Transistors (IGBTs) and MOS-Controlled Thyristors (MCTs).
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The salicide process described in the embodiment above provides a self-aligned silicide contact because of the gate extrusion over the source surface. Since the gate extrusion can be accurately controlled, the spacers formed during the blanket etch process automatically provide a precision alignment for the contact. Tolerance margin is not necessary for the contact mask since the gate extrusion over the source surface can be accurately controlled. Smaller contact openings can be used. The resulting cell size and device on resistance are both reduced. In addition, the silicide on the top of the gate also improves the resistance of poly gate since silicide is a better conductor. Improvement in gate resistance becomes important when cell density increases and gate size becomes narrow.
In the process shown above, the gate is formed prior to the source and/or the body so that the high temperature used in gate formation does not drive the source deeper or increase the required depth of the trench and the gate. In some embodiments, the body or the source or both are formed before the gate is formed.
Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.
Claims
1. A semiconductor device comprising:
- a drain;
- a body in contact with the drain, the body having a body top surface;
- a plurality of source regions embedded in the body, extending downward from the body top surface into the body;
- a trench extending through the plurality of source regions and the body to the drain, said trench having a trench opening and a trench wall; and
- a gate disposed in the trench, said gate having a gate top surface that extends substantially above the body top surface at least in the center region of the trench opening, and said gate having a vertical edge that includes an extended portion extending above the trench opening, the extended portion being substantially aligned with the trench wall; and
- a plurality of spacers insulating the source from the gate, each of the plurality of spacers being situated immediately adjacent to the gate and immediately adjacent to a respective one of the plurality of source regions, wherein each of the plurality of spacers does not substantially extend into the trench and does not substantially extend over the trench;
- a dielectric layer disposed over the source, the spacers, the gate, and at least a portion of the body, the dielectric layer forming a contact opening that has a width that is narrower than a distance between edges of the source regions; and
- a metal layer disposed over the dielectric layer, making a contact with the body at the contact opening.
2. A semiconductor device as recited in claim 1, wherein the gate is insulated by a dielectric layer.
3. A semiconductor device as recited in claim 1, wherein the gate has a side that is substantially perpendicular with respect to the body top surface.
4. A semiconductor device as recited in claim 1, wherein the trench is formed using a SiO2 hard mask.
5. A semiconductor device as recited in claim 1, wherein the trench has a wall formed by having been mutually aligned with the gate.
6. A semiconductor device as recited in claim 1, wherein the trench is mutually aligned with the gate.
7. A semiconductor device as recited in claim 1, wherein the source is no more than 0.5 μm deep.
8. A semiconductor device as recited in claim 1, wherein the gate extends above the body top surface by at least 300 Å.
9. A semiconductor device as recited in claim 1, further comprising a silicide layer connecting the source to the metal layer.
10. A semiconductor device as recited in claim 1, wherein the gate fills the trench opening and extends substantially above the body top surface in approximately all of the trench opening.
11. A semiconductor device as recited in claim 1, wherein the spacer is made of dielectric material.
12. A semiconductor device as recited in claim 9, wherein the gate includes a silicide top layer.
13. A semiconductor device as recited in claim 11, wherein the silicide top layer of the gate is disconnected from the silicide layer connecting the source to the metal contact.
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Type: Grant
Filed: Sep 27, 2004
Date of Patent: Feb 23, 2010
Patent Publication Number: 20060071268
Assignee: Alpha and Omega Semiconductor Limited (Sunnyvale, CA)
Inventors: Sung-Shan Tai (San Jose, CA), Tiesheng Li (San Jose, CA), Anup Bhalla (Santa Clara, CA), Hong Chang (Cupertino, CA), Moses Ho (Campbell, CA)
Primary Examiner: Matthew E Warren
Attorney: Van Pelt, Yi & James LLP
Application Number: 10/952,231
International Classification: H01L 29/94 (20060101);